Magnetic disk files in which information is stored on concentric tracks on one or more magnetizable recording disks are well known, particularly for data processing applications. Information is written on and read from the disks, while they are rotating, by electromagnetic transducing heads supported adjacent the disk surface. At typical state-of-art track densities of, say, 10 tracks/mm, the disk file must be provided with position reference information, which is employed by a head positioning servo system to position and maintain the head precisely over a selected track of the disk. The operation of maintaining the head over a desired track is known as "track following" whereas that of moving the head between tracks is known as "track accessing". Both these operations make use of such position reference information.
In some disk files, the position reference information is provided remotely from the disk surface on which the information or data to be processed is stored e.g. on a dedicated servo disk or disk surface. This has the advantage that position reference information is continuously available. However, at higher track densities, such an arrangement has the disadvantage that it is difficult to guarantee registration between the remote position reference information and the information storage tracks of the disk.
To overcome this disadvantage, it is also known to provide position reference information in sectors, known as "servo sectors", on the information storage surface. These servo sectors are interspersed with "data sectors" containing the stored information and provide accurately registered position reference information on a sampled basis as the disk rotates. The present invention is concerned with this type of sectored arrangement of disk file, early examples of which are described in U.S. Pat. No. 3,185,972 and UK Patent No. 1314695.
A more recent example of a sector servo disk file is described in European published patent application No. 13326. In this patent application, the servo sectors comprise a mark field of radially aligned magnetic transitions, in a unique order, which indicates the start of a servo sector. Following the mark field is a gain field also of radially aligned transitions which is employed for automatic gain control. Following the gain field, is a normal (N) servo field which contains position reference information in the form of a checkerboard pattern of magnetization in which transitions are aligned radially but are of opposite sense in alternate tracks. The tracks of the normal field are the same width as the data tracks but are arranged so that the boundary between them lies on the centre line of the data tracks. This information is read by the transducing head and demodulated to provide a position error signal indicating the position of the head relative to the nearest track centre and, consequently, the "on-track" condition. Since, in the on-track condition, the signal from the head should be zero, this type of servo pattern is sometimes known as a "null" pattern. Following the normal field is a quadrature (Q) field containing an identical null pattern to the normal field but offset from it radially by half a track width so that the tracks of the quadrature pattern are aligned with the data tracks. The quadrature field provides an additional phase shifted position error signal which is employed, together with the normal position error signal, in the access control system for the file.
Both the dedicated and sampled types of system employ the common principle that the position reference information contains contiguous servo tracks of two alternating types whose boundaries each, nominally, coincide with the centre of a data track. Signal contributions from each type of servo track, as detected by the transducing head, are subtracted from one another, either in the head itself or by demodulating circuitry to derive a position error signal from the difference in their amplitudes or areas. This position error signal varies cyclically with radial displacement of the head across the tracks. It is, ideally, linear between slope reversals and is zero when the head lies equally over the boundary of an adjacent pair of servo tracks. In practice, the position error signals derived by these methods are not linear except in a restricted range about zero. The peaks of the cyclic signals, corresponding to maximum off-data track displacement, are rounded and cannot be used as accurate indications of position. This non-linearity becomes more pronounced as track densities are increased.
Unfortunately, however, the need for greater accuracy and linearity of position error signals in sector servo and other disk files increases with increasing track density and decreasing access time. One reason for this is that, to position the head very accurately over the selected track and to minimise off-track deviations, it is necessary to employ the highest possible servo loop gain. Any non-linearity in the fed back position error signal can result in under or over correction and loop instability.
Another reason for greater linearity is the need to combine several portions of the position error signals from adjacent tracks to provide a composite position error signal which is linear over an extended range of several tracks. The provision of such a signal is important in the "settling" period at the end of an access motion since, at high track densities, the access control system can no longer guarantee to position the transducing head within the plus or minus half a track achievable at lower densities. A second need for an extended signal arises in a multiple disk file employing a number of heads nominally on the same "cylinder" on different disk surfaces. Because of misregistration between different disk surfaces, it cannot be guaranteed that heads which are nominally on the same cylinder will actually be located over the corresponding tracks. Thus when switching from one head to another nominally on the same cylinder, an extended linear position error signal is necessary to cope with offsets of more than one track between the heads.
Linearity of the position error signal at all positions of the head over the disk surface is also important in access control systems such as in the system described in European published patent application No. 013326. In that system, the position error signals, derived from servo sectors, are sampled throughout the access motion and compared with a continually available model position error signal of the same general form. The difference between the sampled actual and model position error signals, known as the Access Position Error is fed back to control the access motion. Since only a sampled actual position error signal is available, this signal must be linear whatever the head position on the disk surface when the sampling occurs.
To complete the review of the prior art, reference is made to the following two documents, though it will be noted that neither of these documents explicitly addresses the problem of position error signal linearity discussed above.
UK Patent No. 1566290 shows an alternative sector servo system in which the position error information is in the form of three bursts of high frequency signals which are circumferentially displaced from each other and radially offset by one data track width. Individually, the bursts are two tracks wide but bursts of each of the three types are separated from each other by a single track width. The position of every data track centre line is defined by the boundaries of two of the three bursts. The relative contributions of these two bursts to the head signal are separated in a conventional manner and used to generate a respective one of three position error signals. These three position error signals do not overlap and must be assumed to be linear across their entire range.
Finally, an article entitled "Quad-Burst PES System for Disk File Servo" by W A Herrington and F E Mueller (IBM Technical Disclosure Bulletin Vol. 21, No. 2, July 1978, p. 804) is referred to. This article describes a quadrature burst servo pattern for use in a disk file of the type employing a dedicated servo disk. The servo head width is equal to the burst width which is twice the width of a data track but it is mentioned that the servo head width could be anything between 1 and 3 data track widths. However, no advantage or suggestion of applicability to a sector servo type of disk file is suggested.